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Published online: 11 June 2008 | doi:10.1038/nchina.2008.136

Encryption: Quantum code-breaker

Tim Reid

Introduction

© (2008) istockphoto.com/Geoffrey Holman

Most of our personal data is protected by complex encryption systems such as the widely used RSA algorithm, but these systems may have to change owing to an unexpected threat from quantum physics. Chaoyang Lu at the University of Science and Technology of China in Hefei and co-workers¹ have developed a photonic quantum computer that can quickly crack the RSA code — a task that would take hundreds of years on current supercomputers.

In theory, the huge prime number 'keys' hidden by RSA can be found using a routine called Shor's algorithm. However, Shor's algorithm requires several calculations to be performed at the same time, which is only possible with quantum computers that use quantum bits (qubits), which can be in a superposition of multiple logical states (an entangled state).

Lu and co-workers encoded qubits in entangled pairs of photons, by passing a pulsed ultraviolet laser through a barium borate crystal. By filtering and polarizing the photons they were able to solve Shor's algorithm for the simple case of finding the prime factors of 15 (that is, 3 and 5).

At the same time an almost identical experiment was performed independently at the University of Queensland in Australia², implying that the technique is robust. By learning to manipulate more qubits, researchers could eventually unlock larger numbers and explore an entirely new realm of mathematics.

The authors of this work are from:
Hefei National Laboratory for Physical Sciences at Microscale and Department of Modern Physics, University of Science and Technology of China, Hefei, China; Department of Materials and Department of Physics, Oxford University, Oxford, UK; Physikalisches Institut, Universität Heidelberg, Heidelberg, Germany.

References

1. Lu, C. Y., Browne, D. E., Yang, T. & Pan, J. W. Demonstration of a compiled version of Shor's quantum factoring algorithm using photonic qubits. Phys. Rev. Lett. 99, 250504 (2007). | Article | PubMed | ChemPort |
2. Lanyon, B. P. et al. Experimental demonstration of a compiled version of Shor's algorithm with quantum entanglement. Phys. Rev. Lett. 99, 250505 (2007). | Article | PubMed | ChemPort |